In one known type of nuclear magnetic resonance apparatus, a low temperature superconducting coil supplies a relatively high intensity main DC magnetic field to a sample in a predetermined axial direction; the axial direction is usually vertical and referred to as the Z axis. A pulsed RF field having an appropriate frequency, determined by the DC magnetic field and the nature of the sample, is applied to the sample by a high temperature superconducting probe coil to produce an oscillatory magnetic field along a direction orthogonal to the Z axis. As a result of an interaction between the atoms in the sample, the main DC magnetic field and the RF field, nuclei in the sample precess about the Z axis. After the pulsed RF field is no longer being applied to the sample, the nuclei continue to precess as they return to their original state, inducing a current in the probe coil. The frequencies (or "frequency components") of the current induced in the probe coil is indicative of and determined by the properties of molecules in the sample.
As reported in my co-pending, commonly assigned application Ser. No. 08/965,730, (Varian Docket No. 95-33), entitled "Magnetic Susceptibility Control of Superconducting Materials in Nuclear Magnetic Resonance Probes" filed concurrently herewith, I have discovered that the high temperature superconducting (HTS) probe coil has a tendency to trap a portion of the main DC magnetic field that extends in the Z-axis direction. In the co-pending application Ser. No. 08/965,842, (Varian Docket No. 95-65), entitled "AC Magnetic Susceptibility Control of Superconducting Materials in Nuclear Magnetic Resonance (NMR) Probes" filed concurrently herewith, of which I am a joint inventor, the trapped magnetic field is reduced by applying an AC magnetic field perpendicular to the surface of the HTS probe coil and having a ramped envelope and a relatively low frequency, e.g., 60 Hz. Initially, the applied field is ramped upwardly and then is ramped downwardly. The ramped field applied to the high temperature superconducting probe coil is produced by connecting a 60 Hz source to a further metal coil in close proximity to the probe coil. The field trapping occurs because it is impossible to arrange the high temperature superconducting probe coil so it extends perfectly parallel to the Z axis and because the main DC magnetic field is not homogeneous.
A possible problem with this prior art approach is significant crowding of physical elements in the region where the probe coil is located, bearing in mind that, during NMR normal operation, there is RF coupling between the probe coil and an additional RF metal coil selectively coupled to an RF source and an RF detector. The additional coil and the probe coil must be in close proximity to each other to provide adequate magnetic coupling between them. Crowding is more of a problem when it is realized that, in virtually all configurations, there are two probe coils, on opposite sides of the sample. In addition, a tuning element, such as a paddle, must be provided, which also requires space near the probe coil.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of reducing net DC magnetic flux trapped in a probe coil of a nuclear magnetic resonance device.
Another object of the invention is to provide a new and improved method of and apparatus for reducing net DC magnetic flux trapped in a high temperature superconducting NMR probe coil, wherein the field is reduced with an apparatus that is not necessarily aligned with the coil, but which has an effect on a substrate carrying the coil, and wherein the volume requirements of the structure which enables the field to be reduced are relatively low.